This invention relates generally to the field of oil upgrading and, more particularly, to the stabilization of oil or fractions thereof from at least some of the harmful effects of exposure to light, heat and oxygen, for example.
As petroleum reserves dwindle, crude shale oil and other syncrudes have and will become increasingly important as refinery feedstocks. While in many respects crude shale oil, such as that which results upon the retorting of oil shale, is similar to heavier petroleums, e.g., both have similar hydrogen-to-carbon ratios, they differ in several important aspects. For example, crude shale oils derived from the Green River oil shale deposits of Colorado, Utah, and Wyoming generally have lower sulfur and higher oxygen contents than heavier petroleums. In addition, while crude shale oils typically may contain metals, especially arsenic, which may present some relatively unique refining problems, it is the comparatively high nitrogen loading of crude shale oils that is the principal distinguishing characteristic which makes such shale oils generally unsuitable for use as a conventional refinery feed. For example, typical petroleums generally contain around 0.2 weight percent of nitrogen whereas crude shale oils generally contain in the range of about 1 to about 3 weight percent or more of nitrogen. Also, the nitrogen compounds present in petroleums are generally concentrated in the higher boiling ranges whereas the nitrogen compounds present in crude shale oils are generally distributed throughout the boiling range of the material. Further, the nitrogen compounds in petroleum are predominantly nonbasic compounds, whereas generally about half the nitrogen compounds present in crude shale oils are of a basic nature. Such basic nitrogen compounds are particularly undesirable in refinery feedstocks as such compounds frequently act as severe catalyst poisons. Consequently, crude shale oils, such as those produced upon the retorting of oil shale, generally must be upgraded prior to use as a feedstock that can be commingled with conventional petroleum streams for refining to transportation fuels.
In the view of the problems associated with the presence of nitrogen in oil, particularly syncrude oils, and more particularly crude shale oils, various techniques and procedures for the removal of nitrogen therefrom have been developed. One commonly used technique for nitrogen removal from shale oils is through catalytic hydrotreatment. In such hydrotreatment, crude shale oil and hydrogen are reacted over a catalyst bed at an elevated temperature and pressure to effect olefin and aromatic bond saturation, removal of metals, sulfur, nitrogen and oxygen from the oil, and cleavage of carbon-carbon bonds. These reactions result in the "consumption" of molecular hydrogen by the oil as the hydrogen content of the oil is increased. Typical hydrotreating catalysts used include Ni-Mo, Co-Mo or Ni-W on high surface area, dispersed aluminas. In addition, the catalyst may, for example, be promoted, such as by the addition of P to a Ni-Mo catalyst. Typical catalytic hydrotreating reaction conditions include hydrogen pressures of about 500-3000 psi, operating temperatures of about 600-800.degree. F., and space velocities of about 2 to 0.1 LHSV (liquid volume of oil fed per volume of catalyst per hour). In addition to nitrogen removal, hydrotreatment results in other beneficial or desirable effects such as an increased hydrogen-to-carbon ratio, sulfur and oxygen removal, olefin and aromatic bond removal or saturation and conversion of vacuum residuum hydrocarbons, i.e., hydrocarbons boiling in the 1000+.degree. F. range, to lower boiling range components.
However, hydrotreatment (with the accompanying removal of nitrogen) does not, in and of itself, assure the Stability of the material being treated, e.g., shale oil or particular fractions thereof, such as the "distillate" fraction (i.e., the fraction of the shale oil typically having an initial boiling point in the general range of about 350.degree. F. to about 650.degree. F.), where stability refers to the ability of material to resist discoloration and sediment formation upon exposure to heat, light or oxygen. For example, the presence of both nitrogen and aromatics in a shale oil being processed are believed to contribute to the relative instability of samples of such shale oil as the nitrogen may act to sensitize the aromatics to ultraviolet and/or oxidative induced instability. Furthermore, the severe hydrotreating generally required to obtain shale oil nitrogen levels corresponding to those of typical petroleums frequently results in undesirable processing consequences, such as requiring or resulting in:
1) severe operating conditions, such as high temperatures, hydrogen pressures, or reactor residence times, which conditions and equipment associated therewith are typically relatively costly to obtain, operate and manage;
2) increased production of C.sub.1 to C.sub.4 hydrocarbons from the feedstock;
(3) high hydrogen consumption, in view of the high reaction rates associated with severe hydrotreatment, as hydrogen consumption is believed to increase exponentially with the extent of nitrogen removal; and
4) incapability of using back-mixed, ebullated beds, as it is generally difficult to achieve the high extent of nitrogen removal required by processing dependent on severe hydrotreating through the use of such beds. This despite the fact that ebullated bed type reactors are generally well suited for the treatment of materials, such as inorganic solid contaminated materials, such as shale oils, as ebullated bed reactors are generally well suited to or for: a) removal of organic metals and other fouling reactants; b) handling of the high amounts of heat that accompany hydrotreatment; and c) conversion of 1000.degree. F.+shale oil material (as compared to fixed bed reactors). It is noted, however, that inorganic fine solids, when present in ebullated beds, can cause processing problems such as increased process equipment erosion through abrasion and increased fouling of the catalyst in the reactor.
An alternative technique for the removal of nitrogen from oils, particularly syncrude oils such as crude shale oils, that has been utilized with varying degrees of success is commonly referred to as liquid-liquid (solvent) extraction or selective adsorption. Typically, in such solvent extraction techniques, an incoming liquid mixture such as a synfuel liquid which also contains nonhydrocarbons such as nitrogen compounds, e.g., pyridines, and oxygenated compounds, e.g., phenols, is extracted by a solvent selective for the nonhydrocarbons contained in the synfuel liquid. The removal of nitrogen compounds from a syncrude stream such as raw shale oil, for example, by such extraction alone, however, is generally unlikely to be practical. For example, generally about 50 percent of the oils from aboveground retorts contain nitrogen. Consequently, because such liquid-liquid extraction results in a diminishment in the amount of shale oil recovered thereby, sole reliance on liquid-liquid extraction of nitrogen compounds therefrom will in most cases result in yield losses so severe as to be impractical, e.g., yield losses typically of 50 percent or more. Further, as the amount of solvent required for such extraction will generally be proportional to the quantity of the material to be extracted, typically relatively large quantities of solvent will be required, which in turn will correspondingly increase the cost of solvent recovery and recycle for the process. In addition, effective selective extraction may be difficult to achieve as the nitrogen compounds are of a ubiquitous nature and while raw shale oil generally contains a substantial quantity of nonbasic nitrogen compounds (typically about 1 weight percent or more of the oil), acidic solvents generally tend to be selective for basic nitrogen compounds and are typically relatively ineffective for the extraction of such nonbasic compounds.
U.S. Pat. No. 4,297,206 discloses a method of solvent extraction of synfuel liquids involving an integration of hydrotreatment and extraction. The process disclosed therein involves hydrotreating, rather than recycling directly back to the extractor, the extract resulting upon extraction.
Such a method appears to suffer from at least some of the disadvantages identified above with respect to liquid-liquid (solvent) extraction. For example, large quantities of solvent would appear to be needed for the initial extraction processing. While the use of large quantities of solvent increases the desirability of incorporating some form of solvent recycle and recovery in the process, it would also increase the costs associated therewith. Also, such a technique does not appear to overcome the ubiquitous nature of the nitrogen compounds in the shale oil. Moreover, in such processing only a portion of the shale oil being processed receives the beneficial effects of the hydrotreatment, which follows the extraction processing.